Will your family also be flying back from Las Vegas (or wherever)? In other words, will you be doing a loop road trip?
There is alot to see in CA, OR, and WA. And the West is very popular with RVers. Winging it is possible, with appropriate expectations. I usually call private parks for reservations the day before or the morning of my travel day. Usually works, unless the park is near a Nat'l Park or where there are very few choices. I know I probably won't get into Nat'l Parks or State Parks without prior reservations. Some parks keep a few first come first served sites (not reservable), so you might get lucky if you arrive early in the day on a weekday.
There are alot of public lands in the West. So be sure to research US Forest Service, BLM, etc for campgrounds and boondocking if you don't mind camping without hookups sometimes.
Hwy 395 in CA is a great RV route. Lots to see along 395. You can visit Yosemite from this side of Sierras from Lee Vining (in your tow vehicle). See Mono Lake there too. Advance reservations are definitely necessary at Mono Vista RV Park along 395 near the Yosemite entrance road. There are other options a little farther. Going north I like to cut over at Alturas, take 299-139-39 to Hwy 97 in OR, then continue north. Visit Lava Beds Nat'l Monument on this route.
Crater Lake, Mt. Hood, Columbia River Gorge, then up to WA (Olympic Nat'l Park?). On the way south take 101 along the OR coast then Redwoods in CA. You can take Hwy 20 (north of Ukiah) east across CA to avoid the SF Bay area. Then you can visit Yosemite and Sequoia/Kings Canyon from the west side. Finally back to Las Vegas.
I agree that I-5 and 99 are not pleasant to drive in central CA. But sometimes I just gotta do it to get from point A to point B.
Search the forum for "Oregon Coast" for plenty of reading about things to see and do.
This is just one suggestion of a general route. There are other options (as already posted) and many many sights to see.